This invention relates to sequentially operable control valves for axial flow steam turbines.
Various valve arrangements are used to control the admission of steam into steam turbines. In one arrangement, the bottom wall of the steam chest is formed with a plurality of steam passages leading to steam nozzles. A precision machined seat is mounted in each passage. A valve is movable vertically into and out of engagement with each seat to control the admission of steam from the steam chest through the passages to the nozzles.
As the demand load on the turbine increases, the valves are moved upwardly from their seats in a predetermined sequential order by a valve lift bar which is mounted in the steam chest above the series of seats for movement toward and from the same. The lift bar is formed with an aperture vertically aligned with each valve seat. Each valve has a stem that extends upwardly through an aperture and is slidable in the aperture. The upper end portions of the valve stems are threaded to receive stop nuts. When the lift bar is moved upwardly, it engages the stop nuts on the valve stems, and moves the valves upwardly from their seats. The sequence in which the valves are moved from their seats and the extent of such movement is determined by the position of the stop nut on each valve stem, and jam nuts are provided to lock the stop nuts in adjusted position.
The lift bar is reciprocated vertically in the steam chest by a servomotor which is controlled by a governor, whereby the valves are opened and closed sequentially according to the load demand on the turbine. The sequential, rather than simultaneous, operation is employed to provide for the admission of steam to select groups of nozzles in a predetermined order for more efficient turbine operation and for better balance with minimum vibration. Accordingly, upon initial upward movement of the lift bar, a valve having stop nuts adjusted close to the lift bar will be first moved to open position. The valve of the series having the stop nuts on its stem adjusted to a higher position will open upon further upward movement of the lift bar, and so on. With the lift bar in its uppermost position, all of the valves are open, and as the lift bar is moved downwardly, the valves are closed in reverse sequential order.
The steam passing through the steam chest at high pressure and high velocity and with considerable turbulence imparts vibration of great magnitude in the lift bar and valves. Various solutions have been proposed for the solution of vibration caused problems such as the fatigue and failure of the valve parts, particularly the valve stems.